Why is it important to bleed the air from a hydraulic system?

Published:  27 July, 2020

Air that enters a hydraulic system can cause many problems that could subsequently lead to system failure. Here FPE Seals discusses how to spot these potential problems and why it is so important that air is bled from a system as soon as it is detected.

Essentially, hydraulic pumps are not designed to pump air because when compressed air generates heat. When air contaminates a hydraulic fluid, usually via the pump’s inlet, aeration, cavitation, or foaming can occur. 

Aeration is bad news, as it degrades the hydraulic fluid causing damage to the components of the system due to loss of lubrication, resulting in overheating and burning of the seals. Overheating is particularly dangerous as dieseling can occur when the hydraulic cylinder oil mixes with the air, causing an explosion under compression.

Cavitation, brought on by the rapid changes of pressure in the fluid, causes small vapour-filled bubbles to contaminate the system, which implode when compressed. Ultimately this leads to metal erosion, which harms the system’s components and contaminates the fluid.

Air can also cause foaming in the tank, causing the equipment to operate erratically, as the pump takes large gulps of air along with the oil.

When to bleed a hydraulic system

Abnormal noise is often a tell-tale sign that there is trapped air in a hydraulic system. As air circulates through the system it compresses and decompresses, creating a banging or knocking noise.

This is why abnormal noise must always be checked out and careful inspection given to the condition of the system’s fluid, components and seals, to identify any signs of trapped air or contamination.

It is also important that displacement hydraulic cylinders are bled before installation as any air trapped in the system would work like a gas shock absorber. For this reason, displacement cylinders have a breather at the top, to disperse any air.

And lastly, when testing a new cylinder, it is important to check for potential air contamination, as this can result in blowing the dirt wiper and the hydraulic seal out of its housing extruding past the rod.

All things considered, if spotted early and dealt with quickly, bleeding trapped air in a system, will prevent any long-term damage or operational downtime.


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